When using the destroy option to shut down a nop GEOM module, I/O
operations already in its queue were not being properly drained.
The GEOM framework does the queue draining, but the module needs
to wait for the draining to happen. The waiting is done by adding
a g_nop_providergone() function to wait for the I/O operations to
finish up. This change is similar to change -r345758 made to the
memory-disk driver.

Submitted by: Chuck Silvers
Tested by: Chuck Silvers
MFC after: 1 week
Sponsored by: Netflix